1. Field of the Invention
The present invention relates to backlights of liquid crystal display devices and to liquid crystal display devices in general. More particularly, the present invention relates to a direct type backlight assembly and to a liquid crystal display device for preventing formation of an optical wrinkle caused by heat from a light lamp.
2. Discussion of the Background Art
Recently, research in the area of flat display devices has been focusing on Liquid Crystal Display Devices (LCDs), Field Emission Display Devices (FEDs), Electro-luminescence Display Devices (ELDs), and Plasma Display Panels (PDPs).
In the field of flat display devices, applications pertaining to LCDs are expanding to notebook PC's, desktop monitors, and liquid crystal TVs, because of LCD features of great contrast ratio, suitability for gradation display and motion picture display, and low power consumption.
Referring to FIG. 1, the LCD illustrated therein is provided with an upper substrate 101 having a color filter layer 112 formed thereon for displaying colors, a lower substrate 102 having switching devices for changing an orientation of liquid crystal molecules, and a liquid crystal layer 100 between the upper and lower substrates 101 and 102.
Besides the color filter layer 112, the upper substrate 101 is also provided with a black matrix 111 for blocking a leakage of light, and a first common electrode 113 which is a first electrode for applying a voltage to the liquid crystal layer 100, and besides the thin film transistor 121 (which is a switching device), the lower substrate 102 is also provided with pixel electrode 122 which is a second electrode for receiving a signal from the thin film transistor and applying a voltage to the liquid crystal layer 100.
There are spacers 115 between the upper and lower substrates 101 and 102 for maintaining a fixed gap between the two substrates, and a sealant 116 is provided at a periphery of the substrates for preventing leakage of the liquid crystal material outside of the substrates.
However, since the LCD is a non-emissive type display device which emits no light from itself, a separate external light source is required for illumination. Particularly, in a case of a transmissive LCD, a separate illuminating device, i.e., a backlight assembly, is required in the rear of an LCD panel for emitting and guiding light without fail.
There are a variety of backlight assemblies in use, and they range from a direct type to a light guide plate type. The light guide plate type is provided with a linear light source in tube form, such as a fluorescent lamp (e.g., a hot cathode, or a cold cathode type) at a side of the liquid crystal display panel, for projecting light from the light lamp to an entire surface of the liquid crystal display panel by using a light guide plate, and the direct type is provided with light lamps mounted under the liquid crystal display panel for diffusing light from the light lamps as the light is diffused by a diffuser sheet between the light lamps and the liquid crystal display panel.
However, the light guide plate type is not competitive because of difficult sourcing of components, and the fabrication process has a problem of light leakage caused by close fitting of lamps and the lamp housing, and a drop in luminance caused by loss of light through the light guide plate.
In order to solve these problems, a direct type of light assembly is suggested, in which the light lamps are mounted under the liquid crystal panel, for providing a higher luminance and a more uniform light distribution. A direct type light source is preferable for a display in which uniform luminance is required.
In a case of the direct type, efforts for providing a high luminance backlight assembly has been an object of intense research, particularly directed to the development of a backlight assembly including a plurality of light lamps, or one curved lamp mounted under the display screen.
A background art direct type backlight assembly will be described with reference to the attached drawings. FIG. 2 illustrates a disassembled perspective view of a background art backlight assembly, FIG. 3 illustrates a section across a line I-I′ in FIG. 2, and FIG. 4 illustrates a section of a backlight assembly (for use in describing problems of the background art backlight assembly).
Referring to FIGS. 2 and 3, the background art direct type backlight assembly is provided with a plurality of light lamps 51 for emitting light, a reflective plate 52 for converging the light from the light lamps 51 to a display part of the liquid crystal display panel, optical sheets having a diffuser sheet 55, a prism sheet 56 and a protection sheet 57, and a case 53 for fastening and supporting various components inclusive of the light lamps 51.
In addition to supporting the backlight assembly, the case 53 also supports the liquid crystal display panel above the backlight assembly. There is a bezel part comprised of stainless steel on an outside of the case 53 fitted to an edge of the liquid crystal panel to cover a periphery of the screen, excluding an effective area where a picture is displayed thereon.
Optical sheets are provided for preventing the light lamps from showing on a display surface of the liquid crystal display panel, and providing a light source with a uniform luminance distribution, and includes the diffuser sheet 55, the prism sheet 56, and the protection sheet 57.
The diffuser sheet 55 is formed of a PET (Polyethylene Terephthalate) sheet of approximately 130 μm thick having diffusing particles coated on both sides, and the prism sheet 56 is formed of lenses of approximately 170 μm thick with an apex angle of approximately 90°.
The light lamp is a Cold Cathode Fluorescent Lamp (CCFL) provided with a glass tube having a fluorescent material coated on an inside circumferential surface, and a discharge gas of a mixed inert gas or mercury Hg injected therein, and electrodes at opposite ends connected to power lines 59 and 59a for applying power thereto for driving the lamp, thereby turning it on with an external power source.
The backlight assembly is also provided with a lamp guide 60 fastened to an inside of the case 53 for holding a light lamp 51 with a gap from the reflective plate 52, and support sides 58 fastened to opposite ends of the light lamp 51 for holding and supporting the light lamp 51.
The lamp guide 60 holds the light lamp, and prevents the optical sheets from sagging, and the support sides 58 fixes the light lamp and seats the diffuser sheet 55 thereon.
The backlight assembly provides the light from the light lamp 51 to an information display surface through the diffuser sheet 55, the prism sheet 56, the protection sheet 57, etc. in succession.
However, the background art direct type backlight assembly has problems.
That is, in a case of a monitor or a TV LCD that requires a high luminance, many light lamps are required, which generate heat. As shown in FIG. 4, the optical sheets (particularly the diffuser sheet 55) can expand and sag downward, or form a wrinkle due to heat from the light lamps 51. The wrinkle refracts a path of the light, which results in a dark or bright part on the screen thereby causing a brightness of the screen to be non-uniform.